Fix ParentDimensions so it also delegates margin_* calls

* helps with the rest of #533
* had to turn Dimension margin variables to be nil of not set
  instead of 0 so I could check if a value has been explicitly
  set or not for the dimension
* now better background layouting/more real
* value_modified? necessary to call the super method not just
  blindly return the instance var
good-vjot basically works - no need for the adjusted version anymore